1. Field of the Invention
The present invention relates to an optical semiconductor apparatus usable as optical pickups and optical interconnection, such as light-emitting and light-receiving semiconductor apparatuses provided with monolithically-integrated light-emitting and light-receiving devices, and its fabrication method.
2. Description of the Related Background Art
In the field of the large scale integration (LSI), problems of signal delay in the electric wire, limitation to the transmission distance, and electromagnetic interference (EMI) between electric wires occur. Accordingly, there are limitations to improvements in high-density integration and large capacity. In sharp contrast thereto, the optical interconnection is advantageous in transmission speed, parallel transmission, and EMI-free characteristics. Therefore, the optical interconnection has been researched and developed as promising technology for solving those problems. Further, this technology has been studied to monolithically integrate light-emitting and light-receiving devices on a common substrate for use in the optical interconnection to reduce implementation cost and size.
Particularly, the following light-emitting and light-receiving apparatus is being watched with keen interest since a structure with a fine pitch can be readily fabricated and the degree of its layout is large. In this apparatus, a vertical cavity surface emitting laser (VCSEL) capable of emitting light perpendicularly to a substrate is used as a light-emitting device, a photodiode with a resonator of the same structure as that of the vertical cavity surface emitting laser is used as a light-receiving device, and these devices are monolithically integrated on the substrate. This apparatus, however, has the disadvantage that when the resonator of the surface emitting laser with a very high reflectivity is used in the photodiode without any modification, a wavelength bandwidth of the light-receiving sensitivity of the photodiode is narrowed due to too strong a wavelength sensitivity. Therefore, the reflectivity of the resonator in the photodiode is lowered to solve this problem.
An example is disclosed in xe2x80x9cElectronics Letters, Jun. 20, 1996 Vol. 32 No. 13 pp. 1205-1207xe2x80x9d In this example, as illustrated in FIG. 1, a lower semiconductor multi-layer mirror 1003, an active layer 1005 having a multiple quantum well structure (this layer acts as an absorptive layer in a photodiode region 1000B), and an upper semiconductor multi-layer mirror 1007 are grown on a semiconductor substrate 1001. An upper portion of the upper multi-layer mirror 1007 in the photodiode region 1000B is chemically etched to optimize the light-receiving sensitivity and detection wavelength bandwidth of the photodiode region 1000B, while the upper multi-layer mirror 1007 in a light-emitting device region 1000A remains unchanged.
Another example is disclosed in Japanese Patent Application Laid-Open No. 5(1993)-299689. In this example, as illustrated in FIG. 2, lower semiconductor multi-layer mirrors 1103A and 1103B, an active layer 1105 (this layer acts as an absorptive layer in a photodiode region 1100B), and an upper semiconductor multi-layer mirror 1107 are grown on a semiconductor substrate 1101. A spacer layer 1109 having a predetermined thickness is interposed between the lower semiconductor multi-layer mirrors 1103A and 1103B only in the photodiode region 1100B to adjust the reflectivity of the lower semiconductor multi-layer mirrors 1103A and 1103B and to optimize the light-receiving sensitivity and detection wavelength bandwidth of the light-receiving device region 1100B, while the lower semiconductor multi-layer mirrors 1103A and 1103B in a light-emitting device region 1100A remain unchanged.
In the structure of FIG. 1, although the etching should be stopped at a predetermined location, the etching amount varies at different positions due to an in-surface distribution of the etching speed. A difference in the etching amount influences a phase in the reflection, and hence the light-receiving sensitivity and wavelength bandwidth vary among different devices. Accordingly, it is difficult to obtain devices having the same characteristics and to achieve a good yield.
In the structure of FIG. 2, since the spacer layer 1109 is inserted, regrowth of the spacer layer 1109 and other layers needs to be conducted after growth of the lower multi-layer semiconductor mirror is once interrupted and processes of photolithography, etching, and so forth are performed. Accordingly the fabrication process becomes complicated. Further, devices cannot be freely arranged since locations of light-emitting and light-receiving devices are determined at the stage of the growth.
It is an object of the present invention to provide an optical semiconductor apparatus in which a surface light-emitting device, such as a surface emitting laser and a surface light emitting diode (LED), and a surface light-receiving device having a wide detection wavelength bandwidth are integrated with a flexible combination of these devices, and a method for readily fabricating the optical semiconductor apparatus with good yield.
The present invention is generally directed to an optical semiconductor apparatus provided with a surface light-emitting device and a surface light-receiving device, which includes an active layer, a first spacer layer, and a first semiconductor multi-layer mirror. The active layer, the first spacer layer, and the first semiconductor multi-layer mirror are layered in a layering direction. A first region of the surface light-emitting device and a second region of the surface light-receiving device are arranged in a direction approximately perpendicular to the layering direction, the first region is electrically separated from the second region substantially, and the first spacer layer in the first region and the first spacer layer in the second region are subjected to different oxidization including non-oxidization, respectively, such that resonators composed of the first semiconductor multi-layer mirror and the first spacer layer in the surface light-emitting device and the surface light-receiving device have different wavelength dependencies of reflectivity, respectively. According to such a structure, the optical semiconductor apparatus can be readily achieved with good yield by a selective oxidization performed after a wafer having a desired configuration is fabricated.
Specifically, the first spacer layer is inserted in the first semiconductor multi-layer mirror. The first spacer layer is typically composed of at least a semiconductor layer containing aluminum (Al).
In one case, the first spacer layer in the first region is subjected to non-oxidization, and the first spacer layer in the second region is subjected to oxidization. In this case, the first spacer layer can be composed of a plurality of semiconductor layers, thicknesses and compositions of which are determined such that a resonator composed of the non-oxidized first spacer layer and the first semiconductor multi-layer mirror in the first region is under a non-resonance condition, or creates a maximum reflectivity, for light of a predetermined wavelength. Further, the thicknesses and compositions of the semiconductor layers in the first spacer layer can be determined such that a resonator composed of the oxidized first spacer layer and the first semiconductor multi-layer mirror in the second region is under a resonance condition, or creates a maximum transmissivity, for light of a predetermined wavelength.
In another case, the first spacer layer in the first region is subjected to oxidization, and the first spacer layer in the second region is subjected to non-oxidization. In this case, the first spacer layer can be composed of a plurality of semiconductor layers, thicknesses and compositions of which are determined such that a resonator composed of the oxidized first spacer layer and the first semiconductor multi-layer mirror in the first region is under a non-resonance condition, or creates a maximum reflectivity, for light of a predetermined wavelength. Further, the thicknesses and compositions of the semiconductor layers in the first spacer layer can be determined such that a resonator composed of the non-oxidized first spacer layer and the first semiconductor multi-layer mirror in the second region in under a resonance condition, or creates a maximum transmissivity, for light of a predetermined wavelength.
Typically, the optical semiconductor apparatus further includes a second semiconductor multi-layer mirror which is layered on an opposite side of the first semiconductor multi-layer mirror with respect to the active layer. In this case, a second spacer layer can be layered on an opposite side of the first spacer layer with respect to the active layer. The second spacer layer can also be constructed in the same form as that of the first spacer layer
Further, the optical semiconductor apparatus can include a substrate on which the first semiconductor multi-layer mirror involving the first spacer layer therein, and the active layer are layered in this order, and the first spacer layer in one of the first and second regions is oxidized such that the oxidized first spacer layer can act as an insulating layer.
The present invention is also directed to a fabrication method of fabricating the above optical semiconductor apparatus which includes a step (a) of layering the active layer, the spacer layer, and the semiconductor multi-layer mirror in a layering direction on a substrate, a step (b) of removing the layered layers until sides of the spacer layer are exposed, with the layered layers in the first region and the second region being left, and a step (c) of subjecting the exposed spacer layers in the first region and the second region to different oxidization, respectively.
In one case, in step (c) of the above fabrication method, the spacer layer in the second region is oxidized while the spacer layer in the first region remains non-oxidized. In another case, in step (c) of the above fabrication method, the spacer layer in the first region is oxidized while the spacer layer in the second region remains non-oxidized.
In step (c), the spacer layer in one of the first region and the second region, which is not to be oxidized, may be protected by a protective cover.
In step (a), an area of the second region can be set larger than an area of the first region, and in step (c), the spacer layers in the first region and the second region can be treated in a common oxidization atmosphere without using a protective cover.
In step (a), the semiconductor multi-layer mirror involving the spacer layer can be layered on a side of the substrate with respect to the active layer, and in step (c), the spacer layer in one of the first region and the second region can be completely oxidized so as to function as an insulating layer.
The principle of the present invention will be described. In general, a highly-reflective semiconductor multi-layer mirror for light of a predetermined wavelength can be formed by sequentially and alternately layering low-refractive-index layers and high-refractive-index layers each of which has a thickness corresponding to an odd-number multiple of a quarter wavelength. In such a multi-layer mirror, the phase of light reflected at an interface between the low-index layer and the high-index layer coincides with that of light reflected at an interface between the high-index layer and the low-index layer. A high reflectivity of the multi-layer mirror can be thus achieved.
A case where a layer in the above semiconductor multi-layer mirror is replaced by a spacer layer is considered. The spacer layer can have either a single-layer structure or a plural-layer structure, but the latter can be designed more easily. The reflectivity can be decreased for the predetermined wavelength when the following condition, under which a resonator comprised of the multi-layer mirror and the spacer layer resonates, is satisfied for this wavelength. In this condition, reflection by the multi-layer mirror above the spacer layer for incident light from the active layer is in an opposite phase to reflection by the multi-layer mirror below the spacer layer for incident light from the active layer, and these reflections weaken each other. This condition is referred to as the resonance condition of the resonator in this specification.
In contrast, the reflectivity can be maximized for the predetermined wavelength when the following condition, under which a resonator comprised of the multi-layer mirror and the spacer layer does not resonate, is satisfied for this wavelength. In this condition, reflection by the multi-layer mirror above the spacer layer for incident light from the active layer is in a consonant phase with reflection by the multi-layer mirror below the spacer layer for incident light from the active layer, and these reflections strengthen each other. This condition is referred to as the non-resonance condition of the resonator in this specification.
In the optical semiconductor apparatus of the present invention, typically, the spacer layer has an Al-containing single-layer or plural-layer structure, the spacer layer in a certain region is oxidized to change its refractive index in this region, and the surface light-emitting device, such as a vertical cavity surface emitting laser (VCSEL), is formed in a high-reflectivity region while the surface light-receiving device is formed in a low-reflectivity region.
A characteristic matrix of a layer in the spacer layer can be represented by the following matrix,       M    i    =                    (                                                            cos                ⁢                                  xe2x80x83                                ⁢                                  δ                  i                                                                                                      j                  ⁢                                      xe2x80x83                                    ⁢                  sin                  ⁢                                      xe2x80x83                                    ⁢                                      δ                    i                                                                    n                  i                                                                                                                          jn                  i                                ⁢                sin                ⁢                                  xe2x80x83                                ⁢                                  δ                  i                                                                                    cos                ⁢                                  xe2x80x83                                ⁢                                  δ                  i                                                                    )            ⁢              xe2x80x83            ⁢              δ        i              =                  2        ⁢        π        ⁢                  xe2x80x83                ⁢                  n          i                ⁢                  d          i                    λ      
where xcex is the wavelength, ni is the refractive index of the layer, and di is the thickness of the layer. A characteristic matrix Mspace of the spacer layer is given by a product of the characteristic matrices Mi of layers in the spacer layer. Where thicknesses and refractive-indices of the layers in the spacer layer are set such that the (1, 2) element of the characteristic matrix Mspace is equal to zero (0), the resonator comprised of the semiconductor multi-layer mirror and the spacer layer is under the resonance condition. In the description of the (1, 2) element, the first numeral (here 1) indicates a row number, and the second numeral (here 2) indicates a column number. Where thicknesses and refractive-indices of the layers in the spacer layer are set such that the (1, 1) element of the characteristic matrix Mspace is equal to zero (0), the resonator comprised of the semiconductor multi-layer mirror and the spacer layer is under the non-resonance condition.
As a typical example, a three-layer spacer layer of AlGaAs (its Al mole fraction is 0.9, and its refractive index is 3.07), AlAs (its refractive index is 3.0), and AlGaAs (its Al mole fraction is 0.9, and its refractive index is 3.07) is considered. Such a spacer layer consisting of odd-number layers can be readily constructed since its structure can be easily symmetric. When a semiconductor layer containing Al, such as AlGaAs, is thermally oxidized, oxidization of the layer proceeds faster as the Al mole fraction increases. In the case of a layer whose Al mole fraction is less than 0.9, the layer cannot be easily oxidized. Therefore, when the above three-layer spacer layer is subjected to oxidization treatment, only the AlAs layer is oxidized and changed to AlxOy to lower its refractive index from 3.0 to 1.6. In contrast thereto, the AlGaAs layer with the Al mole fraction of 0.9 remains unchanged.
Where a surface emitting laser is formed in a region having a non-oxidized spacer layer and a surface light-receiving device is formed in a region having an oxidized spacer layer, it is only necessary to determine the thicknesses of layers in the spacer layer such that the (1, 1) element of the characteristic matrix of the non-oxidized spacer layer is equal to zero (0) and the (1, 2) element of the characteristic matrix of the oxidized spacer layer is equal to zero (0).
Where the spacer layer is composed of three layers of AlGaAs (its Al mole fraction is 0.9, and its refractive index is 3.07), AlAs (its refractive index is 3.0), and AlGaAs (its Al mole fraction is 0.9, and its refractive index is 3.07), the above thicknesses only need to be 528 xc3x85, 995 xc3x85 and 528 xc3x85 for a wavelength of 0.83 xcexcm. Naturally, the structure (number of layers, thickness of each layer, and refractive index of each layer) of the spacer layer is not limited to the above example.
FIG. 5 shows the wavelength dependency of absorptivity of a resonator-type surface light-receiving device in which the spacer layer regulated as discussed above is inserted. Where the spacer layer is not oxidized (no oxidization) (i.e., the light-receiving device contains a spacer layer having the same layer structure as that of the surface emitting laser), the absorptivity only has a peak of less than 10% and the detection wavelength bandwidth is narrow, as is seen from FIG. 5. In contrast, where the spacer layer is oxidized (oxidization present), the absorptivity can be more than 50% and the detection wavelength bandwidth can be widened, e.g., 6 nm to 7 nm. Further, as can be seen from FIG. 5, when the insertion position of the spacer layer in the semiconductor multi-layer mirror is altered, the resonator construction consisting of the semiconductor multi-layer mirror and the spacer layer is also changed, resulting in a change in the detection wavelength spectrum of the light-receiving device. In contrast, when the spacer layer satisfies the above-discussed condition relevant to the resonator, the reflectivity and resonance spectrum of the surface emitting laser remain unchanged even if the insertion position of the spacer layer in the semiconductor multi-layer mirror of the surface emitting laser is altered. Accordingly, it is possible by altering the insertion position of the spacer layer to change the performance (light-receiving sensitivity, detection wavelength bandwidth, and the like) of the light-receiving device while the performance (threshold current, oscillation wavelength, and the like) of the light-emitting device remains unchanged. The freedom in designing can be thus increased.
Further, it is also possible to form a surface light-receiving device in a region having a non-oxidized spacer layer and form a surface emitting laser in a region having an oxidized spacer layer. In this case, it is only necessary to determine the thicknesses of layers in the spacer layer such that the (1, 2) element of the characteristic matrix of the non-oxidized spacer layer is equal to zero (0) and the (1, 1) element of the characteristic matrix of the oxidized spacer layer is equal to zero (0). Here, where the spacer layer is composed of three layers of AlGaAs (its Al mole fraction is 0.9, and its refractive index is 3.07), AlAs (its refractive index is 3.0), and AlGaAs (its Al mole fraction is 0.9, and its refractive index is 3.07), the above thicknesses only need to be 190 xc3x85, 995 xc3x85 and 190 xc3x85 for a wavelength of 0.83 xcexcm.
Additionally, it is easy to insert a plurality of spacer layers in the semiconductor multi-layer mirror such that desired detection wavelength spectrum and oscillation wavelength spectrum can be obtained in the light-emitting and light-receiving apparatus. Furthermore, at least a spacer layer can be readily inserted in each of the semiconductor multi-layer mirrors provided on opposite sides of the active layer.
In conclusion, according to the optical semiconductor apparatus of the present invention, the spacer layer is designed such that characteristic matrices of different portions of the spacer layer (these portions are subjected to different oxidization treatments including non-oxidization) in the surface light-emitting device and the light-receiving device achieve desired oscillation wavelength spectrum and desired detection wavelength spectrum, respectively, and the surface light-emitting and light-receiving devices are integrated in a desired combination with good yield.
In the optical semiconductor apparatus of the present invention, the following advantages can be attained.
(1) A surface light-emitting device, such as a vertical cavity surface emitting laser, with a low threshold and a surface light-receiving device, such as a photodiode, with a wide detection wavelength bandwidth can be integrated on a common substrate.
(2) An appropriate highly-reflective multi-layer mirror in a surface light-emitting device and an appropriate multi-layer mirror with a reduced-reflectivity in a surface light-receiving device can be formed by a collective simultaneous growth of semiconductor material, so the fabrication process can be facilitated.
(3) Locations of a surface light-emitting device and a surface light-receiving device can be freely set without any restriction on a semiconductor-grown wafer, so an optical semiconductor apparatus can be obtained with a high versatility.
(4) Resonance spectra of a surface light-emitting device and a surface light-receiving device can be determined at the stage of semiconductor growth since no etching of a mirror surface is needed. Therefore, characteristics of the respective devices can be uniformly set, and the yield of an optical semiconductor apparatus can be improved.
(5) Resonance spectra and light-receiving sensitivity of a surface light-emitting device and a surface light-receiving device can be flexibly designed by appropriately inserting at least a spacer layer (single-layer structure or plural-layer structure) in a semiconductor multi-layer mirror.
These advantages, as well as others will be more readily understood in connection with the following detailed description of the preferred embodiments and examples of the invention in connection with the drawings.